General purpose colour assessment is typically carried out in an illumination cabinet that includes a light source in its upper part. The sample may be placed on the floor of the cabinet, or a flat sample may be fixed to a planar or curved inclined surface for ease of viewing. The front of the cabinet may be open for viewing by eye or the cabinet may be closed with the sample being viewed via an instrument such as a digital camera in the interior. In more specialist applications, such as continuous production lines, different arrangements may be in place for colour assessment and most aspects of the present invention remain relevant to those as will be evident to the skilled reader.
In order to allow viewing of the sample under different lighting conditions, it is often possible to switch between multiple light sources, each designed to matched a different reference illuminant. To allow the effective visual comparison of a sample under different illuminants, the switching must be almost instantaneous. The size of the cabinet limits the number of light sources that can be accommodated and hence the number of reference illuminants that can be approximated.
The light sources for known colour assessment cabinets are typically either incandescent or fluorescent.
Incandescent sources use a high temperature filament that produces light having a continuous spectrum of the same general form as black body radiation, with most power at the red end of the spectrum. Natural daylight and the corresponding reference illuminants for daylight have most power in the green and blue parts of the spectrum, so the incandescent lamps have to be run at high power to provide sufficient green and blue light. In an essentially subtractive process, the majority of the red and orange light is then filtered out to match the desired reference illuminant, with the result that energy is wasted and the cabinet can become excessively hot.
Fluorescent sources use a voltage discharge to excite rarefied gas in a sealed tube. The inside of the tube is coated with phosphors that are thereby caused to fluoresce, each in a narrow band of wavelengths. In an essentially additive process, the outputs of the different phosphors combine to build up an appropriate distribution of power across the desired range of wavelengths. Fluorescent sources are energy efficient but their spectra tend to be rather discontinuous because of the narrow band emissions of the phosphors. For colour assessment applications, the number of phosphors is typically increased compared with fluorescent tubes for domestic or commercial use to mimic as closely as possible a reference illuminant. However, increasing the number of phosphors adds to the cost of the lamps. A further problem is that the phosphors decay unevenly with time, especially at blue wavelengths, so that the spectral power distribution of the light source changes and the lamp has to be discarded and replaced after a period much less than its normal life would be in less colour-sensitive applications. Moreover, ordinary start-up circuits for fluorescent bulbs cause a delay before the bulb illuminates therefore, to provide the instant switching between sources that is necessary for visual colour comparison of samples, specialist and expensive start-up circuits need to be provided.
The invention in its broadest aspect uses light-emitting diodes (LEDs) to compensate for deficiencies in a fluorescent light source during colour assessment. LEDs are reasonably energy efficient and are available in a wide variety of colours with both broad and narrow bands of wavelengths. They have the great advantage over additional phosphors in the fluorescent lamp that their intensity can be adjusted during use, including the choice to switch off certain of the LEDs entirely (i.e. zero intensity). Thus the LEDs may be used in the following ways:                To reduce the number of phosphors required in the fluorescent lamp, by filling gaps in the spectrum of the lamp.        To prolong the useful life of the lamp or improve the colour constancy of the light source over the lifetime of the lamp, by compensating for changes in the lamp's phosphors as they age.        To allow a light source to approximate multiple reference illuminants using a single, basic fluorescent lamp, by using the LEDs in different combinations to compensate for the differences between the basic lamp and each of the reference illuminants. This saves on space and cost and increases the flexibility of the cabinet, which can be “reprogrammed” for new light sources without requiring new hardware. The intensity of the LEDs can be changed instantly while the basic fluorescent lamp remains on continuously, therefore it is possible to switch instantly between different illuminants without providing a specialist start-up circuit for the lamp.        
The spectrum of light that falls on the sample is determined not only by the spectrum that the light source emits but also by changes in the spectrum when the light from the source is reflected from the cabinet walls. If two cabinets have differently coloured walls, LEDs in their respective light sources can further be used to compensate for that difference and allow the cabinets to be used for consistent colour assessment, for example by two parties at different locations.
If the sample is not to be viewed directly but through an instrument such as a digital camera, that instrument will have its own spectral response, i.e. the way its sensitivity to light varies with wavelength. The past approach has been to illuminate the sample using light that approximates a reference illuminant, to image the sample using the camera, and then to use image processing software to compensate for the difference between the spectral responses of the camera and the human eye. The invention provides an alternative approach, which is to change the illumination of the sample so that the way the camera images the sample under the changed illumination approximates the way the human eye perceives the sample under the reference illuminant. For example, if the camera is less sensitive to blue light, then the relative intensity of blue LEDs in the light source may be increased compared with the reference spectrum.
The control of intensity of the LEDs may be achieved by varying the applied current or voltage to change the intensity of light continuously emitted from them; or it may preferably be achieved by pulse width modulation of the current through the LEDs to change the average intensity of the emitted light, the pulses being at sufficiently high frequency that they are not noticeable to the observer or observing instrument.
Although the present invention is described with reference to light-emitting diodes, it is clear that it would be equally applicable to other light-emitting semiconductor devices. If the wavelength of such devices were tunable, that would provide a further way of controlling the spectral power distribution of the light source.
Although the invention is described with reference to a linear fluorescent tube, it would work equally well with other, less common shapes.